1. Field of the Invention
The present invention relates to an exhaust purification device and exhaust purification method of an internal combustion engine.
2. Description of the Related Art
As a catalyst for purifying NOx contained in exhaust gas when fuel is burned under a lean air-fuel ratio, there is known a catalyst comprised of a carrier made of alumina on the surface of which a layer of a NOx absorbent comprised of an alkali metal or alkali earth is formed and on the surface of which a precious metal catalyst such as platinum is carried (for example, see Japanese Unexamined Patent Publication (Kokai) No. 6-108826). In this catalyst, when the catalyst is activated and the air-fuel ratio of the exhaust gas is lean, the NOx contained in the exhaust gas is absorbed in the NOx absorbent, while when the air-fuel ratio of the exhaust gas is made rich, the NOx which had been absorbed in the NOx absorbent is released and reduced.
However, this NOx absorption and release action is believed not to be performed when the catalyst is not activated. Therefore, in the internal combustion engine described in the above publication, when the catalyst is not activated, the catalyst is heated by an electric heater.
The inventors researched catalysts designed to perform this NOx absorption and release action and as a result discovered that while the nitrogen monoxide contained in exhaust gas is not absorbed in the NOx absorbent when the catalyst is not activated, nitrogen dioxide contained in exhaust gas is stored in the NOx absorbent even when the catalyst is not activated.